Extracting metals and equilibria
The reactivity series
The reactivity series orders metals by how readily they lose electrons (form positive ions). From most to least reactive:
K > Na > Ca > Mg > Al > C > Zn > Fe > Sn > Pb > H > Cu > Ag > Au > Pt
Carbon and hydrogen are non-metals included for reference (carbon can reduce metal oxides; hydrogen is the reference for acids).
Extraction methods depend on reactivity
| Position in reactivity series | Extraction method | Example |
|---|---|---|
| Above carbon (very reactive) | Electrolysis | Al (from Al₂O₃), Na, Ca, K |
| Below carbon (less reactive) | Reduction by carbon (coke) | Fe (from Fe₂O₃ in blast furnace), Zn |
| Below hydrogen (unreactive) | Found native / heat alone | Cu (from CuO + H₂ or C), Ag, Au |
Reduction with carbon (iron in the blast furnace)
Carbon (coke) is added to iron ore (Fe₂O₃). Carbon first burns to form CO₂, then CO: C + O₂ → CO₂ CO₂ + C → 2CO Carbon monoxide reduces iron oxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂ This is reduction because iron oxide loses oxygen / Fe³⁺ gains electrons.
Aluminium by electrolysis
Aluminium cannot be extracted by carbon because Al is above C in the reactivity series (Al would not be reduced). Instead, purified aluminium oxide (alumina, Al₂O₃) is dissolved in molten cryolite (lowers melting point from ~2000°C to ~850°C) and electrolysed: Cathode: Al³⁺ + 3e⁻ → Al Anode: 2O²⁻ → O₂ + 4e⁻ (carbon anodes oxidised/burned away → replaced regularly)
Displacement reactions
A more reactive metal displaces a less reactive metal from its salt solution: Fe + CuSO₄ → FeSO₄ + Cu (iron is above copper → displaces it) OIL RIG: Fe is oxidised (Fe → Fe²⁺ + 2e⁻); Cu²⁺ is reduced (Cu²⁺ + 2e⁻ → Cu).
Dynamic equilibrium
A reversible reaction can proceed in both the forward and reverse direction, shown with the ⇌ symbol.
At dynamic equilibrium: the forward and reverse reactions occur at the same rate; concentrations of reactants and products remain constant; the system is closed.
Le Chatelier's principle
If a system at equilibrium is subjected to a change, the position of equilibrium shifts to oppose that change:
| Change | Effect on equilibrium |
|---|---|
| Increase temperature | Shifts toward the endothermic direction |
| Decrease temperature | Shifts toward the exothermic direction |
| Increase pressure | Shifts toward fewer moles of gas |
| Decrease pressure | Shifts toward more moles of gas |
| Increase concentration of reactant | Shifts right (toward products) |
| Add a catalyst | No shift — reaches equilibrium faster, no change in yield |
Example: the Haber process (N₂ + 3H₂ ⇌ 2NH₃, forward reaction exothermic)
Higher pressure → more NH₃ (fewer gas moles on right). Higher temperature → less NH₃ (shifts left, endothermic direction). A catalyst (iron) is used to reach equilibrium faster, not to change the position of equilibrium.
⚠Common mistakes
- Catalysts do NOT change equilibrium position: they speed up both forward and reverse reactions equally.
- Equilibrium position vs rate: high temperature speeds up the reaction but may reduce yield (if forward is exothermic).
- Carbon cannot reduce aluminium: Al is more reactive than C — electrolysis is required.
- Confusing oxidation and reduction in displacement: the more reactive metal is oxidised; the less reactive ion is reduced.
AI-generated · claude-opus-4-7 · v3-edexcel-chemistry